A number of drug delivery devices are known in which medicament is driven from a reservoir, under the action of a driving mechanism, through a needle and into the skin of a patient. A problem with known devices is that the delivery rate accuracy suffers when the volume of drug is small. Such inaccuracies arise in many cases from the driving mechanisms employed which give rise to variations in delivery rates. For example, where a gas is generated to drive a plunger in a cartridge or vial, the volume of gas depends in part on the temperature of the environment. The variation in volume will also depend on the total amount of gas already present in the chamber.
The reason that gas generation is preferred over mechanical driving mechanisms is that the design of gas generating cells, such as electrolytic cells' is extremely simple when compared to mechanical equivalents, and this provides significant advantages in terms of reliability and cost-effectiveness. Systems are known in which a mechanically driven ratchet is used to incrementally deliver fixed amounts of medicament, but such systems can be expensive to manufacture. In particular, the accuracy of delivery of small amounts of drug depends on the manufacturing tolerances of the ratchet mechanism. For mass-produced, moulded, cut or pressed ratchets, the tolerances may not be sufficiently accurate to deliver the required small volumes, which means that more expensive manufacturing techniques are required to obtain the necessary tolerances. Such considerations are particularly important if the devices are intended to be disposable, in which case a low unit cost is required without compromising accuracy or reliability or system performance.
A problem with gas driven mechanisms, however, is that it is extremely difficult to ensure that a gas chamber is leakproof without taking elaborate manufacturing and quality control precautions. Even if a leak is minor and relatively slow, this poses a real problem when the mechanism is supposed to accurately deliver small volumes over extended timespans. Thus, for gas generation systems, it is preferred to design a system that is leak free (which is costly and typically more complex) or provide a system that functions accurately in spite of minor or relatively slow leaks. In the alternative, gas generation may not be suitable for lower delivery rates. As mentioned above, mechanical equivalents having the required precision (e.g. clockwork mechanisms) are overly expensive and complex for incorporation into inexpensive devices which may be disposable.
For many drug delivery regimes, it is desirable to provide both steady state delivery (“basal delivery”) and instantaneous bursts of drug (“bolus delivery”) as required. In particular, in patient controlled analgesia or PCA, it may be advantageous to provide a continuous basal infusion of drug for chronic pain treatment, supplemented to a certain extent by bolus delivery. The bolus delivery would be activated by the patient to deal with increased temporary pain levels (“break-through pain”), with safeguards being incorporated to prevent overdosing.
Another area in which precisely controlled dosing can be particularly indicated is in chronotherapeutic drug delivery, in which the drug delivery rate varies over time. Most notably, diurnal or circadian rhythms cause variations in the amounts of certain drugs required by a patient during a 24-hour period. This is most notably required to combat variations in disease and/or condition effects throughout a 24-hour cycle.
For example, hypertension crises, angina, and sudden cardiac death are most likely to occur in the morning, whereas sickle cell crises and perforated ulcer crises are most likely to occur in the afternoon. The concept of chronotherapeutics is discussed in more detail in an article by Smolensky & Labrecque, Pharmaceutical News 4, No. 2, 1997, pp. 10-16. The discussion in this article is principally in terms of conventional oral dosing of drugs to take account of chronotherapeutic variations in drug uptake, effects, and requirements, but many of the principles are applicable to other delivery routes. Circadian rhythm applications would also apply to hormonal therapies.
Accordingly there is a need to provide a drug delivery device capable of regulating drug delivery dosages to provide increased dosages at the times when such dosages are more likely to be required. This gives rise to a need for a device in which the delivery rate is accurately controllable over a wide range of delivery rates. In general, devices which are designed to deliver small amounts of drug are not particularly suitable for high drug delivery rates without being specifically adapted in this regard, and vice versa. Moreover there is a need to provide such a device that is relatively compact so that it is fixed to the user during use and disposed of when the treatment is finished. Such a device must be also relatively inexpensive to manufacture yet maintain accurate and reliable delivery rates
The present invention aims to provide improved drug delivery devices in which smaller volumes of liquid can be delivered more accurately than in prior art devices, thereby giving rise to overall more controlled delivery rates. The invention also aims to provide such devices which additionally allow higher delivery rates to be provided on demand, up to and including bolus delivery. Moreover, the present invention provides for a drug delivery device wherein the technology used to provide for accurate delivery rates is relatively easy and inexpensive to manufacture. Further, the present invention employs designs for the gas generating system and delivery system so that space within the device is minimised and parts used within the device are easy and inexpensive to manufacture while maintaining high tolerances. In addition, the present invention provides for a certain amount of gas leakage while delivering accurate dosages. This eliminates the need for costly sealing devices and systems which increase cost and decrease reliability in the event of gas leakage.